Systems and Methods for Magnetic Field-Dependent Relaxometry Using Magnetic Resonance Imaging

ABSTRACT

Systems and methods for magnetic field-dependent relaxometry using magnetic resonance imaging (“MRI”] are provided. Relaxation parameters, including longitudinal relaxation time (“T1”) and transverse relaxation time (“T2”), are estimated from magnetic resonance signal data acquired at multiple different magnetic field strengths using the same MRI system. By measuring these relaxation parameters as a function of magnetic field strength, T1 dispersion data, T2 dispersion data, or both, are generated. Based on this dispersion data, quantitative physiological parameters can be estimated. As one example, iron content can be estimated from T2 dispersion data.

BACKGROUND OF THE INVENTION

The field of the invention is systems and methods for magnetic resonanceimaging (“MRI”). More particularly, the invention relates to systems andmethods for magnetic field-dependent relaxometry using MRI.

The use of MRI as a quantitative tool has continued to attract greatinterest in the clinical and research communities. One of the mostcommon MRI techniques for quantitative diagnosis is relaxometry, inwhich longitudinal relaxation time, T₁, transverse relaxation time, T₂,or both can be estimated in a region-of-interest and used to generatequantitative maps based on the relaxation times. In general, relaxometrytechniques make use of sampling magnetic resonance signals at two ormore different time points (e.g., echo times) using a long repetitiontime (“TR”).

Imaging iron content in the body (and especially the brain) is verydesirable. Iron content has been shown to correlate with multipleneurological disorders (e.g., Parkinson's and Alzheimer's disease).Current methods to image iron content revolve around quantifying thedrop in magnetic resonance signal when the echo time of a gradient echosequence is increased. This allows the apparent transverse relaxationrate, R₂*=1/T₂*, to be measured in vivo. However, R₂* can change formany reasons that might not be associated to iron content. Furthermore,this technique is not very sensitive to small quantities of iron.

Longitudinal and transverse relaxation times have been shown to bedependent on magnetic field strength. For instance, the transverserelaxation times, T₂, of ferritin solutions show a linear increase withmagnetic field strength, with the slope of this increase depending onthe ferritin loading factor. If the T₂ of tissues at multiple fieldstrengths can be computed, then iron content can be estimated. Moreover,it may be possible to determine which form the iron is deposited in,which could be very advantageous in trying to identify iron's role inneurological disorders.

These magnetic field-dependent relaxometry techniques have not beenadopted for routine clinical implementation, however, because theyrequire estimating the relaxation parameters at multiple differentmagnetic field strengths. Using currently available technology, thisrequirement is generally satisfied only by moving the subject betweenmultiple different MRI systems, each with a different magnetic fieldstrength (e.g., a 0.5 T, 1.0 T, 1.5 T, and 3 T system.

Thus, there remains a need for providing systems and methods formagnetic field-dependent relaxometry that can be readily implemented inclinical and research environments without the need for multipledifferent MRI systems.

SUMMARY OF THE INVENTION

The present invention overcomes the aforementioned drawbacks byproviding a method for magnetic field-dependent relaxometry usingmagnetic resonance imaging (“MRI”). The method includes acquiring firstdata from a subject using an MRI system having a main magnetic field ata first magnetic field strength by sampling a first magnetic resonancesignal at a first plurality of time points. The main magnetic field ofthe MRI system is then adjusted to a second magnetic field strength andsecond data are acquired from the subject using the MRI system while themain magnetic field of the MRI system is at the second magnetic fieldstrength by sampling a second magnetic resonance signal at a secondplurality of time points. A first value of a relaxation parameter isestimated by fitting the first data to a signal model that describesmagnetic resonance signal relaxation as a function of the relaxationparameter, and a second value of the relaxation parameter is estimatedby fitting the second data to the signal model. Dispersion data are thengenerated by associating the first value of the relaxation parameterwith the first magnetic field strength and the second value of therelaxation parameter with the second magnetic field strength.

It is another aspect of the present invention to provide A method forproducing a map of a quantitative physiological parameter in a region ina subject using MRI. Magnetic resonance signals are generated in theregion using the MRI system, and a data set is acquired from the regionusing the MRI system by sampling the magnetic resonance signalsgenerated in the region. These steps are repeated a plurality of timesto acquire a plurality of data sets. Each of the data sets is acquiredat a different magnetic field strength by adjusting the magnetic fieldstrength of the main magnetic field of the MRI system before generatingthe magnetic resonance signals in the region. Values of a relaxationparameter are estimated in the region by fitting each of the pluralityof data sets to a signal model that describes magnetic resonance signalrelaxation as a function of the relaxation parameter. Dispersion dataare then generated for each location in the region by associatingestimated values of the relaxation parameter with the magnetic fieldstrength at which the data set used to estimate the values of therelaxation parameter was acquired. A map of a quantitative physiologicalparameter in the region is generated by computing the quantitativephysiological parameter at each location in the region from thedispersion data.

The foregoing and other aspects and advantages of the invention willappear from the following description. In the description, reference ismade to the accompanying drawings that form a part hereof, and in whichthere is shown by way of illustration a preferred embodiment of theinvention. Such embodiment does not necessarily represent the full scopeof the invention, however, and reference is made therefore to the claimsand herein for interpreting the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart setting forth the steps of an example method formagnetic field-dependent relaxometry using a magnetic resonance imaging(“MRI”) system, and for optionally generating maps of quantitativephysiological parameters based on dispersion data.

FIG. 2 is a block diagram of an example MRI system that can implementthe method of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Described here are systems and methods for magnetic field-dependentrelaxometry using magnetic resonance imaging (“MRI”). Relaxometrygenerally refers to the measurement of relaxation parameters, includinglongitudinal relaxation time (“T1”) and transverse relaxation time(“T2”). By measuring these relaxation parameters as a function ofmagnetic field strength, T1 dispersion data, T2 dispersion data, orboth, can be obtained. Based on this dispersion data, quantitativephysiological parameters can be estimated. As one example, iron contentcan be estimated from T2 dispersion data.

Currently, relaxation dispersion data can be obtained by imaging asubject in a number of different MRI systems, each with a differentmagnetic field strength. This approach has several limitations. Forinstance, the approach requires access to several different MRI systems,each with different field strengths, which may not be available at mostclinical sites. Also, because the subject must be moved between multipledifferent MRI systems, the images of the subject from the different MRIsystems must be co-registered before dispersion data can be generatedfrom them.

The systems and methods of the present invention, however, utilize asingle MRI system that can be operated to rapidly ramp its main magneticfield strength, thereby allowing for measurements of relaxationparameters at multiple different field strengths without having to movethe subject.

Referring now to FIG. 1, a flowchart is illustrated as setting forth thesteps of an example method for measuring relaxation parameter dispersionusing MRI.

The method includes directing the MRI system to perform a pulse sequencethat acquires data by sampling a magnetic resonance signal at varioustime points, as indicated at step 102. In general, the magneticresonance signal is generated by nuclear spins relaxing back toequilibrium and thus can include a free induction decay (“FID”) signal,a gradient echo signal, a spin echo signal, a stimulated echo signal, orany other suitable magnetic resonance signal. In general, the MRI systemis operated to generate magnetic resonance signals across a region ofthe subject, such as an image slice, image slab, image volume, or otherspatially localized region-of-interest

It will be appreciated by those skilled in the art that the choice ofpulse sequence will influence the type of magnetic resonance signal thatis formed and also the relaxation parameter to be studied. For instance,an inversion recovery or T1-weighted pulse sequence may be used forexamining longitudinal relaxation, whereas a T2-weighted pulse sequencemay be used for examining transverse relaxation.

A determination is made at decision block 104 whether data have beenacquired at a desired number of different magnetic field strengths. Ifnot, then the magnetic field strength is adjusted, as indicated at step106, and additional data are acquired at the new magnetic fieldstrength. Thus, the method includes acquiring data at least at a firstmagnetic field strength and a second magnetic field strength; however,the process can be generally repeated to acquire a plurality of datasets at each of a plurality of different magnetic field strengths.Accordingly, in a preferred MRI system, a tunable RF coil isimplemented, such that the RF coil can be tuned to the appropriateresonance frequencies associated with the different magnetic fieldstrengths. Likewise, a broadband receiver can preferably be implementedto provide a wide range of resonance frequencies, and thus fieldstrengths, that can be used in a single scan. Examples of such tunableRF coils and broadband receivers will be appreciated by those skilled inthe art.

In one specific embodiment, the magnetic field is adjusted using an MRIsystem that is capable of rapidly ramping up or down its main magneticfield. As example of such a system is described in co-pending PCTApplication Serial No. PCT/IB2015/057979. With this type of system, themain magnetic field can be ramped to different strengths depending onthe amount of applied current. Advantageously, the main magnetic fieldin can be ramped in a clinically reasonable amount of time. As oneexample, the main magnetic field can be ramped from zero to 0.5 T inabout ten minutes. As another example, the main magnetic field can beramped from 0.5 T to 0.4 T in about one minute or less. Thus, in someembodiments, the magnetic field strength of the main magnetic field canbe ramped between a first and second value in less than fifteen minutes,and in some embodiments less than two minutes.

Using such a system, then, the main magnetic field can be incrementallyadjusted to acquire data at multiple different field strengths in aclinically reasonable span of time, thereby providing magneticfield-dependent relaxometry. As one example, the main magnetic field canbe adjusted in increments of 0.1 T; however, it will be appreciated thatother increments greater or less than 0.1 T can also be used (e.g., 0.05T, 0.2 T, 0.25 T, 0.5 T). In some embodiments, the main magnetic fieldcan be further adjusted using a coil insert to modulate the localmagnetic field in the MRI system.

When data have been acquired at all of the desired magnetic fieldstrengths, the data are fit to the appropriate signal models to extractrelaxation parameters as a function of magnetic field strength, asindicated at step 108. In general, the signal model used is a signalmodel that describes the relaxation of magnetic resonance signals as afunction of a relaxation parameter, such as transverse relaxation time,transverse relaxation rate, apparent transverse relaxation time,apparent transverse relaxation rate, longitudinal relaxation time, orlongitudinal relaxation rate. This step may include reconstructingimages from the acquired data and fitting the image magnitude, imagephase, or both, to the appropriate signal model. As one example, imagemagnitudes at the various sampled time points can be fit to a signalmodel based on the Bloch equations, which relate measured signalintensity to one or both of the relaxation time parameters, T1 and T2.As another example, the signal model can be based on an exponentialdecay model for estimating T2, or a recovery signal model for estimatingT1. As a result of this step, values of the relaxation parameters areestimated for each magnetic field strength at which data were acquired.In some embodiments, maps of the relaxation parameters can be generated,which depict the spatial distribution of the relaxation parameters in aregion of the subject, such as the imaged field-of-view.

Dispersion data can then be generated by associating the estimatedrelaxation parameters with the respective magnetic field strengths, asindicated at step 110. As one example, the dispersion data can includecurves or other plots that describe relaxation parameters as a functionof magnetic field strength.

Using the dispersion data, other physical parameters associated with thesubject being examined can be computed, as indicated at step 112. As ageneral example, quantitative physiological parameters can be estimatedfrom the dispersion data. As one non-limiting example, iron content intissue can be computed from T2 dispersion data. As another non-limitingexample, albumin content can be estimated in applications where acontrast agent, such as Vasovist® (also known as Albavar® manufacturedby Lantheus Medical Imaging Inc.) is bound to albumin. In someinstances, albumin content can be measured using a delta relaxationenhanced magnetic resonance (“dreMR”) technique. Other parameters thatcan be measured from dreMR contrast can also be estimated. In general,it will be appreciated by those skilled in the art, however, that anysuitable physical or physiological parameters associated with one ormore relaxation parameters can be estimated or computed.

Maps of these parameters can be generated to depict the spatialdistribution of the estimated parameters in the imaged field-of-view, asindicated at step 114. These maps can be useful for identifying regionsin the subject that have certain physical characteristics attributableto the estimated parameters. For instance, in the example above, themaps can be useful for identifying regions in the subject that havevarying degrees of iron content.

Example MRI System

Referring now to FIG. 2, an example of an MRI system 10 that is capableof rapidly ramping its magnetic field is illustrated. The MRI system 10generally includes a magnet assembly 12 for providing a magnetic field14 that is substantially uniform within a bore 16 that may hold asubject 18 or other object to be imaged. The magnet assembly 12 supportsa radio frequency (“RF”) coil (not shown) that may provide an RFexcitation to nuclear spins in the subject 18 or object positionedwithin the bore 16. The RF coil communicates with an RF system 20producing the necessary electrical waveforms, as is understood in theart In some embodiments, the RF coil can be a tunable RF coil that canbe tuned to various different resonance frequencies (e.g., resonancefrequencies associated with different magnetic field strengths), as isunderstood in the art. The RF system 20 can, in some embodiments,include a broadband receiver capable of receiving magnetic resonancesignals across a broad range of resonance frequencies, thereby allowinga similarly broad range of different magnetic field strengths to beimplemented.

The magnet assembly 12 also supports three axes of gradient coils (notshown) of a type known in the art, and which communicate with acorresponding gradient system 22 providing electrical power to thegradient coils to produce magnetic field gradients, G_(x), G_(y), andG_(z) over time.

A data acquisition system 24 connects to RF reception coils (not shown)that are supported within the magnet assembly 12 or positioned withinbore 16.

The RF system 20, gradient system 22, and data acquisition system 24each communicates with a controller 26 that generates pulse sequencesthat include RF pulses from the RF system 20 and gradient pulses fromgradient system 22. The data acquisition system 24 receives magneticresonance signals from the RF system 20 and provides the magneticresonance signals to a data processing system 28, which operates toprocess the magnetic resonance signals and to reconstruct imagestherefrom. The reconstructed images can be provided to a display 30 fordisplay to a user.

The magnet assembly 12 includes one or more magnet coils 32 housed in avacuum housing 34, which generally provides a cryostat for the magnetcoils 32, and mechanically cooled by a mechanical cryocooler 36, such asa Gifford-McMahon (“GM”) cryocooler or a pulse tube cryocooler. In oneexample configuration, the cryocooler can be a Model RDK-305Gifford-McMahon cryocooler manufactured by Sumitomo Heavy Industries(Japan). In general, the cryocooler 36 is in thermal contact with themagnet coils 32 and is operable to lower the temperature of the magnetcoils 32 and to maintain the magnet coils 32 and a desired operatingtemperature. In some embodiments the cryocooler 36 includes a firststage in thermal contact with the vacuum housing 34 and a second stagein thermal contact with the magnet coils 32. In these embodiments, thefirst stage of the cryocooler 36 maintains the vacuum housing 34 at afirst temperature and the second stage of the cryocooler 36 maintainsthe magnet coils 32 at a second temperature that is lower than the firsttemperature.

The magnet coils 32 are composed of a superconducting material andtherefore provide a superconducting magnet The superconducting materialis preferably selected to be a material with a suitable criticaltemperature such that the magnet coils 32 are capable of achievingdesired magnetic field strengths over a range of suitable temperatures.As one example, the superconducting material can be niobium (“Nb”),which has a transition temperature of about 9.2 K. As another example,the superconducting material can be niobium-titanium (“NbTi”), which hasa transition temperature of about 10 K. As still another example, thesuperconducting material can be triniobium-tin (“Nb₃Sn”), which has atransition temperature of about 18.3 K.

The choice of superconducting material will define the range of magneticfield strengths achievable with the magnet assembly 12. Preferably, thesuperconducting material is chosen such that magnetic field strengths inthe range of about 0.0 T to about 3.0 T can be achieved over a range oftemperatures that can be suitably achieved by the cryocooler 36. In someconfigurations, however, the superconducting material can be chosen toprovide magnetic field strengths higher than 3.0 T.

The cryocooler 36 is operable to maintain the magnet coils 32 at anoperational temperature at which the magnet coils 32 aresuperconducting, such as a temperature that is below the transition, orcritical, temperature for the material of which the magnet coils 32 arecomposed. As one example, a lower operational temperature limit can beabout 4 K and an upper operational temperature limit can be at or nearthe transition, or critical, temperature of the superconducting materialof which the magnet coils 32 are composed.

The current density in the magnet coils 32 in the MRI system 10 iscontrollable to rapidly ramp up or ramp down the magnetic field 14generated by the magnet assembly 12 while controlling the temperature ofthe magnet coils 32 with the cryocooler 36 to keep the temperature belowthe transition temperature of the superconducting material of which themagnet coils 32 are composed. As one example, the magnetic field 14 canbe ramped up or ramped down on the order of minutes, such as fifteenminutes or less.

In general, the current density in the magnet coils 32 can be increasedor decreased by connecting the magnet coils 32 to a circuit with a powersupply 38 that is in electrical communication with the magnet coils 32via a switch 40 and operating the power supply 38 to increase ordecrease the current in the connected circuit. The switch 40 isgenerally a superconducting switch that is operable between a first,closed, state and a second, open, state.

When the switch 40 is in its open state, the magnet coils 32 are in aclosed circuit, which is sometimes referred to as a “persistent mode.”In this configuration, the magnet coils 32 are in a superconductingstate so long as the temperature of the magnet coils 32 is maintained ata temperature at or below the transition temperature of thesuperconducting material of which they are composed.

When the switch 40 is in the closed state, however, the magnet coils 32and the power supply 38 can be placed in a connected circuit, and thecurrent supplied by the power supply 38 and the current in the magnetcoils 32 will try to equalize. For instance, if the power supply 38 isoperated to supply more current to the connected circuit, the current inthe magnet coils 32 will increase, which will increase the strength ofthe magnetic field 14. On the other hand, if the power supply 38 isoperated to decrease the current in the connected circuit, the currentin the magnet coils 32 will decrease, which will decrease the strengthof the magnetic field 14.

It will be appreciated by those skilled in the art that any suitablesuperconducting switch can be used for selectively connecting the magnetcoils 32 and power supply 38 into a connected circuit; however, as onenon-limiting example, the switch 40 may include a length ofsuperconducting wire that is connected in parallel to the magnet coils32 and the power supply 38. To operate such a switch 40 into its closedstate, a heater in thermal contact with the switch 40 is operated toraise the temperature of the superconducting wire above its transitiontemperature, which in turn makes the wire highly resistive compared tothe inductive impedance of the magnet coils 32. As a result, very littlecurrent will flow through the switch 40. The power supply 38 can then beplaced into a connected circuit with the magnet coils 32. When in thisconnected circuit, the current in the power supply 38 and the magnetcoils 32 will try to equalize; thus, by adjusting the current suppliedby the power supply 38, the current density in the magnet coils 32 canbe increased or decreased to respectively ramp up or ramp down themagnetic field 14. To operate the switch 40 into its open state, thesuperconducting wire in the switch 40 is cooled below its transitiontemperature, which places the magnet coils 32 back into a closedcircuit, thereby disconnecting the power supply 38 and allowing all ofthe current to flow through the magnet coils 32.

When the magnet coils 32 are in the connected circuit with the powersupply 38, the temperature of the magnet coils 32 will increase as thecurrent in the connected circuit equalizes. Thus, the temperature of themagnet coils 32 should be monitored to ensure that the temperature ofthe magnet coils 32 remains below the transition temperature for thesuperconducting material of which they are composed. Because placing themagnet coils 32 into a connected circuit with the power supply 38 willtend to increase the temperature of the magnet coils 32, the rate atwhich the magnetic field 14 can be ramped up or ramped down will dependin part on the cooling capacity of the cryocooler 36. For instance, acryocooler with a larger cooling capacity will be able to more rapidlyremove heat from the magnet coils 32 while they are in a connectedcircuit with the power supply 38.

The power supply 38 and the switch 40 operate under control from thecontroller 26 to provide current to the magnet coils 32 when the powersupply 38 is in a connected circuit with the magnet coils 32. A currentmonitor 42 measures the current flowing to the magnet coils 32 from thepower supply 38, and a measure of the current can be provided to thecontroller 26 to control the ramping up or ramping down of the magneticfield 14. In some configurations, the current monitor 42 is integratedinto the power supply 38.

A temperature monitor 44 is in thermal contact with the magnet assembly12 and operates to measure a temperature of the magnet coils 32 inreal-time. As one example, the temperature monitor 44 can include athermocouple temperature sensor, a diode temperature sensor (e.g., asilicon diode or a GaAlAs diode), a resistance temperature detector(“RTD”), a capacitive temperature sensor, and so on. RTD-basedtemperature sensors can be composed of ceramic oxynitride, germanium, orruthenium oxide. The temperature of the magnet coils 32 is monitored andcan be provided to the controller 26 to control the ramping up orramping down of the magnetic field 14.

In operation, the controller 26 is programmed to ramp up or ramp downthe magnetic field 14 of the magnet assembly 12 in response toinstructions from a user. As mentioned above, the magnetic field 14 canbe ramped down by decreasing the current density in the magnet coils 32by supplying current to the magnet coils 32 from the power supply 38 viathe switch 40, which is controlled by the controller 26. Likewise, thestrength of the magnetic field 14 can be ramped up by increasing thecurrent density in the magnet coils 32 by supplying current to themagnet coils 32 from the power supply 38 via the switch 40, which iscontrolled by the controller 26.

The controller 26 is also programmed to monitor various operationalparameter values associated with the MRI system 10 before, during, andafter ramping the magnetic field 14 up or down. As one example, asmentioned above, the controller 26 can monitor the current supplied tothe magnet coils 32 by the power supply 38 via data received from thecurrent monitor 42. As another example, as mentioned above, thecontroller 26 can monitor the temperature of the magnet coils 32 viadata received from the temperature monitor 44. As still another example,the controller 26 can monitor the strength of the magnetic field 14,such as by receiving data from a magnetic field sensor, such as a Hallprobe or the like, positioned in or proximate to the bore 16 of themagnet assembly 12.

One or more computer systems can be provided with the MRI system 10 forprocessing acquired data in accordance with the methods described above.As one example, the data processing system 28 can be used to process theacquired data.

For example, the data processing system 28 can receive magneticresonance data from the data acquisition system 24 and processes it inaccordance with instructions downloaded from an operator workstation.Such processing may include those methods described above forreconstructing images, fitting signals to signal models, generatingdispersion data, and computing quantitative or physical parameters fromdispersion data.

Images reconstructed by the data processing system 28 can be conveyedback to the operator workstation for storage, and real-time images canbe stored in a memory, from which they may be output to display 30.

The MRI system 10 may also include one or more networked workstations.By way of example, a networked workstation may include a display; one ormore input devices, such as a keyboard and mouse; and a processor. Thenetworked workstation may be located within the same facility as the MRIsystem 10, or in a different facility, such as a different healthcareinstitution or clinic.

The networked workstation, whether within the same facility or in adifferent facility as the MRI system 10, may gain remote access to thedata processing system 28 via a communication system. Accordingly,multiple networked workstations may have access to the data processingsystem 28. In this manner, magnetic resonance data, reconstructedimages, or other data may be exchanged between the data processingsystem 28 and the networked workstations, such that the data or imagesmay be remotely processed by a networked workstation. This data may beexchanged in any suitable format, such as in accordance with thetransmission control protocol (“TCP”), the internet protocol (“IP”), orother known or suitable protocols.

The present invention has been described in terms of one or morepreferred embodiments, and it should be appreciated that manyequivalents, alternatives, variations, and modifications, aside fromthose expressly stated, are possible and within the scope of theinvention.

1. A method for magnetic field-dependent relaxometry using magneticresonance imaging (MRI), the steps of the method comprising: (a)acquiring first data from a subject using an MRI system having a mainmagnetic field at a first magnetic field strength by sampling a firstmagnetic resonance signal at a first plurality of time points; (b)adjusting the main magnetic field of the MRI system to a second magneticfield strength; (c) acquiring second data from the subject using the MRIsystem while the main magnetic field of the MRI system is at the secondmagnetic field strength by sampling a second magnetic resonance signalat a second plurality of time points; (d) estimating a first value of arelaxation parameter by fitting the first data to a signal model thatdescribes magnetic resonance signal relaxation as a function of therelaxation parameter; (e) estimating a second value of the relaxationparameter by fitting the second data to the signal model; and (f)generating dispersion data by associating the first value of therelaxation parameter with the first magnetic field strength and thesecond value of the relaxation parameter with the second magnetic fieldstrength.
 2. The method as recited in claim 1, wherein: step (d) isrepeated for multiple locations in a field-of-view from which the firstdata are acquired to estimate first values of the relaxation parameterat each of the multiple locations; step (e) is repeated for multiplelocations in the field-of-view from which the second data are acquiredto estimate second values of the relaxation parameter at each of themultiple locations; and the dispersion data generated in step (f)associates the first values of the relaxation parameter at each of themultiple locations with the first magnetic field strength and the secondvalues of the relaxation parameter at each of the multiple locationswith the second magnetic field strength.
 3. The method as recited inclaim 1, wherein the main magnetic field of the MRI system is adjustedby ramping the main magnetic field of the MRI system from the firstmagnetic field strength to the second magnetic field strength.
 4. Themethod as recited in claim 3, wherein the main magnetic field of the MRIsystem is ramped from the first magnetic field strength to the secondmagnetic field strength in less than fifteen minutes.
 5. The method asrecited in claim 4, wherein the main magnetic field of the MRI system isramped from the first magnetic field strength to the second magneticfield strength in less than two minutes.
 6. The method as recited inclaim 1, wherein a difference between the first magnetic field strengthand the second magnetic field strength is about 0.1 Tesla.
 7. The methodas recited in claim 1, wherein the relaxation parameter is one of atransverse relaxation time or a transverse relaxation rate.
 8. Themethod as recited in claim 7, further comprising estimating from thedispersion data, a physical parameter associated with the subject
 9. Themethod as recited in claim 8, wherein the physical parameter is anestimate of iron content in the subject.
 10. The method as recited inclaim 1, wherein the relaxation parameter is one of a longitudinalrelaxation time or a longitudinal relaxation rate.
 11. A method forproducing a map of a quantitative physiological parameter in a region ina subject using magnetic resonance imaging (MRI), the steps of themethod comprising: (a) generating magnetic resonance signals in theregion using the MRI system; (b) acquiring a data set from the regionusing the MRI system by sampling the magnetic resonance signalsgenerated in the region; (c) repeating steps (a) and (b) a plurality oftimes to acquire a plurality of data sets, each of the data sets beingacquired at a different magnetic field strength by adjusting themagnetic field strength of the main magnetic field of the MRI systembefore generating the magnetic resonance signals in the region; (d)estimating values of a relaxation parameter in the region by fittingeach of the plurality of data sets to a signal model that describesmagnetic resonance signal relaxation as a function of the relaxationparameter; (e) generating dispersion data for each location in theregion by associating values of the relaxation parameter estimated instep (d) with the magnetic field strength at which the data set used toestimate the values of the relaxation parameter was acquired; and (f)generating a map of a quantitative physiological parameter in the regionby computing the quantitative physiological parameter at each locationin the region from the dispersion data.
 12. The method as recited inclaim 11, wherein the main magnetic field of the MRI system is adjustedin step (c) by ramping the main magnetic field of the MRI system from afirst magnetic field strength to a second magnetic field strength. 13.The method as recited in claim 12, wherein the main magnetic field ofthe MRI system is ramped from the first magnetic field strength to thesecond magnetic field strength in less than fifteen minutes.
 14. Themethod as recited in claim 13, wherein the main magnetic field of theMRI system is ramped from the first magnetic field strength to thesecond magnetic field strength in less than two minutes.
 15. The methodas recited in claim 12, wherein a difference between the first magneticfield strength and the second magnetic field strength is about 0.1Tesla.
 16. The method as recited in claim 11, wherein the relaxationparameter is one of a transverse relaxation time or a transverserelaxation rate.
 17. The method as recited in claim 16, wherein thequantitative physiological parameter is iron content.
 18. The method asrecited in claim 1, wherein the relaxation parameter is one of alongitudinal relaxation time or a longitudinal relaxation rate.